I'm terrible when it comes to remembering birthdays. To correct this, I've put myself reminders in my address book and even tried using an application in Facebook.
But it sill doesn't seem to work for me.
It's regularly updated and only recently has changed to a much nicer Leopard friendly interface. You use it to browse all your contacts, alphabetically, by date or age.True, its purpose is pretty limited, but at least Birthdaybook will save you the hassle of entering birthdays one by one in your calendar.